Interdecadal Dynamics of the North Pacific Ocean

Abstract

An isopycnal ocean model forced by NCEP?NCAR reanalysis wind stresses and heat fluxes is used to study the interdecadal variability of the Pacific Ocean in the 1958?97 period. A reasonable agreement is found between the model's modes of variability and those obtained by other researchers from both 100 years of observations and theoretical predictions. In agreement with previous observational work, decadal and interdecadal timescales have different descriptions, and from this study it is suggested that they indeed have different dynamics. This study focuses on the dynamics of the ocean's interdecadal variability, that is, of timescales of about 20 yr. The decadal timescale, that is, 10 yr, is briefly outlined and compared with previous studies. It is found that atmospheric heat fluxes play a key role in establishing the interdecadal SST pattern in the midlatitudinal North Pacific. These fluxes would excite a high baroclinic mode, igniting a series of events that move around the basin. In midlatitudes, interdecadal SSTs are most sensitive to the heat flux forcing along the eastern boundary north of about 30°N, in the western North Pacific at about 40°N, and along 20°N eastward of the date line; in the eastern North Pacific and north of 40°N, interdecadal pycnocline anomalies move across the Gulf of Alaska and toward the Aleutian Islands up to about the Kamchatka Peninsula, continuing to the southwest down to about 28°N. In their path, pycnocline oscillations induce SST changes in the Kuroshio?Oyashio Extension. On the other hand, in the eastern Tropics, the wind stress curl would induce interdecadal pycnocline oscillations that (between 10° and 20°N) propagate as Rossby waves, similar to those observed there for annual and interannual timescales, which, after crossing the date line, turn toward the north-northwest. All of these waves and/or events move or propagate within areas where the mean flow is of smaller amplitude than the phase speed in the direction of motion. In addition, the results presented here would suggest that a process similar to a servomechanism, and as envisaged by other authors, is present along 40°N, suggestive of an active ocean?atmosphere interaction over this area. Major differences are found between decadal and interdecadal dynamics.